Electrochemical cells, including proton exchange membrane fuel cells, sensors, electrolyzers, and electrochemical reactors, are known in the art. Typically, the central component of such a cell is a membrane electrode assembly (MEA), comprising two catalyzing electrodes separated by an ion-conducting membrane (ICM). Fuel cells incorporating an MEA structure offer the potential for high output density, are driveable at a reasonable temperature, and exhaust mainly CO2 and water. Fuel cells are viewed as potential, clean energy sources for motor vehicles, marine craft, aircraft, portable electronic devices such as notebook computers and cell phones, toys, tools and equipment, spacecraft, buildings, components of these, and the like. When an MEA of a fuel cell incorporates a central polymeric membrane, the fuel cell may be referred to as a polymer electrolyte fuel cell (PEFC). Examples of MEA's and their use in fuel cells are further described in U.S. Pat. Nos. 6,756,146; 6,749,713; 6,238,534; 6,183,668; 6,042,959; 5,879,828; and 5,910,378.
In fuel cells, hydrogen gas, or a fuel gas including hydrogen, is fed to a fuel electrode (anode) and oxygen or a gas such as air including oxygen is fed to an oxidizer electrode (cathode). Hydrogen is oxidized as a result, generating electricity. Typically, catalysts are used at one or both of the anode and cathode to facilitate this reaction. Common electrode catalysts include platinum or platinum used in combination with one or more of palladium, rhodium, iridium, ruthenium, osmium, gold, tungsten, chromium, manganese, iron, cobalt, nickel, copper, alloys or intermetallic compositions of these, combinations thereof, or the like.
The hydrogen used by a fuel cell may be obtained by reforming one or more hydrogen-containing fuels, e.g., an alcohol or hydrocarbon. Examples of reforming processes include steam reforming, autothermal reforming, and partial-oxidation reforming. Ideally, the products of reformation would include only hydrogen and carbon dioxide. In actual practice, carbon monoxide is also a reformation by-product, and water and nitrogen often are present as well. By way of example, a typical reformed gas might include 45 to 75 volume percent hydrogen, 15 to 25 volume percent carbon dioxide, up to 3 to about 5 volume percent water, up to 3 to about 5 volume percent nitrogen, and 0.5 to 2 volume percent carbon monoxide. The carbon monoxide unfortunately has a tendency to poison the platinum catalyst used in fuel cells, significantly reducing fuel cell output.
In order to avoid catalyst poisoning, it is desirable to reduce the CO content of the reformed gas to no more than about 10 ppm to about 100 ppm. However, the low boiling point and high critical temperature of CO make its removal by physical adsorption very difficult, particularly at room temperature.
One feasible method for removing carbon monoxide from reformed gas generally has involved using a catalytic system that selectively oxidizes the CO relative to hydrogen, converting the CO to carbon dioxide [CO+½O2=>CO2]. After this catalytic conversion, the reformed gas may be supplied directly to a fuel cell inasmuch as the carbon dioxide formed is much less harmful to the fuel cell catalyst, e.g., platinum. The process of selectively oxidizing CO relative to hydrogen is known as selective oxidation or preferential oxidation (PROX) and is a highly active area of research. The desirable characteristics of such a catalyst have been described by Park et al [Journal of Power Sources 132 (2004) 18-28] as including the following:
(1) high CO oxidation activity at low temperatures;
(2) good selectivity with respect to the undesired oxidation of H2;
(3) a wide temperature window for a greater than 99% conversion of CO; and
(4) tolerance towards the presence of CO2 and H2O in the feed.
CO oxidation activity may be expressed as percentage CO conversion (XCO) and is calculated as follows:
      X    CO    =                                          [            CO            ]                    in                -                              [            CO            ]                    out                                      [          CO          ]                in              ×    100    ⁢                  ⁢    percent  Selectivity towards CO(SCO) is defined as the ratio of the O2 used for CO oxidation to total O2 consumption. SCO is computed as a percentage as follows:
      S    CO    =                                          [            CO            ]                    in                -                              [            CO            ]                    out                            2        ×                  (                                                    [                                  O                  2                                ]                            in                        -                                          [                                  O                  2                                ]                            out                                )                      ×    100    ⁢                  ⁢    percent  Good PROX catalysts are both highly active and highly selective. Another important parameter is the stoichiometric oxygen excess factor lambda, λ, wherein λ=2*[O2]/[CO]. When λ=1, this means that oxygen is present in the stoichiometric amount for complete CO oxidation. When λ>1, this corresponds to an oxygen excess over that required for complete CO oxidation. It is preferable in fuel cell operation to keep λ as low as possible while still maintaining >99.5% CO conversion. This minimizes dilution of the hydrogen fuel and usually maximizes the selectivity of the PROX catalyst.
Considerable effort has been applied in the industry to design suitable catalysts capable of this kind of selective oxidation. Many significant challenges are faced. As one challenge, many conventional CO catalysts have insufficient activity and/or selectivity under reasonable operating conditions. For instance, many CO oxidation catalysts are only active at temperatures of 150° C. or higher, where selectivity may be inadequate. This means that not only carbon monoxide but also hydrogen is oxidized [H2+½O2=>H2O], wasting the hydrogen fuel. Even if some degree of selectivity is shown by a catalyst operating at such higher temperatures, the catalytically processed gas might have to be cooled before the gas is supplied to a fuel cell.
It would be much more desirable to have a selective CO catalyst that functions at lower temperatures, e.g., below about 70° C., or even below about 40° C., or even more desirably at room temperature or below. Very few CO oxidation catalysts, though, are active and/or selective at such low temperatures. This is true even though oxidation to CO2 is thermodynamically favored. Additionally, some catalysts are damaged or otherwise inhibited in the presence of CO2 and/or water, both typically being present in a reformed gas. Other catalysts are limited by a short service and/or shelf life.
Most of the proposed catalysts for selective oxidation of carbon monoxide in hydrogen-rich streams have been alumina supported platinum group metals (especially Pt, Rh, Ru, and Ir). Supported Pt catalysts exhibit a maximum activity for CO oxidation at around 200° C. with fair selectivities in the range from 40-60%. High conversion at lower temperatures requires more oxygen in the feed (high λ). This lowers the selectivity even further.
A report by Cominos et al. [Catalysis Today 110 (2005) 140-153] describes a Pt—Rh on γ-alumina catalyst that was able to reduce 1.12% CO to 10 ppm in a single stage reactor at 140-160° C. with an inlet oxygen to carbon monoxide ratio of 4 (λ=8). However, selectivity under these conditions was only 12.5% resulting in extensive loss of hydrogen fuel.
Low temperature activity can be improved by using titania, ceria or ceria-zirconia supports or by promotion with base metals like cobalt and iron; but selectivity is usually less than 50%.
In the absence of H2O and CO2, base metal catalysts such as CuO—CeO2 have been shown to be at least as active for PROX as the supported platinum group metals and considerably more selective. However, these catalysts are adversely affected by the presence of CO2 and H2O in the reformate gas stream [Bae et al., Catalysis Communications 6 (2005) 507-511]. This effect is often quite large. Catalyst activity can be restored by operation at a higher temperature, but this decreases selectivity.
It has been observed that nanogold on iron oxide can be made to be active for selective CO oxidation. See, e.g., Landon et al. (2005) Chem. Commun., “Selective Oxidation of CO in the presence of H2, H2O, and CO2 Via Gold For Use In Fuel Cells,” 3385-3387.
At ambient to sub-ambient temperatures, the best gold catalysts are considerably more active for CO oxidation than the most active promoted platinum group metal catalysts known. Gold is also considerably cheaper than platinum. Catalytically active gold, though, is quite different from the platinum group metal catalysts discussed above. The standard techniques used in the preparation of supported platinum group metal catalysts give inactive CO oxidation catalysts when applied to gold. Different techniques, therefore, have been developed for deposition of finely divided gold on various supports. Even so, highly active gold catalysts have been difficult to prepare reproducibly. Scaleup from small lab preparations to larger batches has also proved difficult.
These technical challenges have greatly hindered the industrial application of gold catalysts. This is unfortunate since the very high activities of gold catalysts for CO oxidation at ambient and sub-ambient temperatures and their tolerance for high water vapor concentrations make them otherwise strong candidates for use in applications in which oxidation of CO would be desired.
Because ultra-fine particles of gold generally are very mobile and possess large surface energies, ultra-fine particles of gold tend to sinter easily. This tendency to sinter makes ultrafine gold hard to handle. Sintering also is undesirable inasmuch as the catalytic activity of gold tends to fall off as its particle size increases. This problem is relatively unique to gold and is much less of an issue with other noble metals such as platinum (Pt) and palladium (Pd). Thus, it is desired to develop methods to deposit and immobilize ultra-fine gold particles on a carrier in a uniformly dispersed state.
Known methods to deposit catalytically active gold on various supports recently have been summarized by Bond and Thompson (G. C. Bond and David T. Thompson, Gold Bulletin, 2000, 33(2) 41) as including (i) coprecipitation, in which the support and gold precursors are brought out of solution, perhaps as hydroxides, by adding a base such as sodium carbonate; (ii) deposition-precipitation, in which the gold precursor is precipitated onto a suspension of the pre-formed support by raising the pH, and (iii) Iwasawa's method in which a gold-phosphine complex (e.g., [Au(PPh3)]NO3) is made to react with a freshly precipitated support precursor. Other procedures such as the use of colloids, grafting and vapor deposition, have met with varying degrees of success.
These methods, however, suffer from difficulties aptly described by Wolf and Schüth, Applied Catalysis A: General, 2002, 226 (1-2) 1-13 (hereinafter the Wolf et al. article). The Wolf et al. article states that “[a]lthough rarely expressed in publications, it also is well known that the reproducibility of highly active gold catalysts is typically very low.” The reasons cited for this reproducibility problem with these methods include the difficulty in controlling gold particle size, the poisoning of the catalyst by ions such as Cl, the inability of these methods to control nano-sized gold particle deposition, the loss of active gold in the pores of the substrate, the necessity in some cases of thermal treatments to activate the catalysts, inactivation of certain catalytic sites by thermal treatment, the lack of control of gold oxidation state, and the inhomogeneous nature of the hydrolysis of gold solutions by the addition of a base.
In short, gold offers great potential as a catalyst, but the difficulties involved with handling catalytically active gold have severely restricted the development of commercially feasible, gold-based, catalytic systems.
German Patent Publication DE 10030637 A1 describes using PVD techniques to deposit gold onto support media. The support media described in this document, though, are merely ceramic titanates made under conditions in which the media would lack nanoporosity. Thus, this document fails to indicate the importance of using nanoporous media to support catalytically active gold deposited using PVD techniques. International PCT Patent Publications WO 99/47726 and WO 97/43042 provide lists of support media, catalytically active metals, and/or methods for depositing the catalytically active metals onto the support media. These two documents, however, also fail to appreciate the benefits of using nanoporous media as a support for catalytically active gold deposited via PVD. Indeed, WO 99/47726 lists many preferred supports that lack nanoporosity.
Relatively recently, very effective, heterogeneous catalyst systems and related methodologies using catalytically active gold have been described in assignee's co-pending United States patent application having U.S. Ser. No. 10/948,012, titled CATALYSTS, ACTIVATING AGENTS, SUPPORT MEDIA, AND RELATED METHODOLOGIES USEFUL FOR MAKING CATALYST SYSTEMS ESPECIALLY WHEN THE CATALYST IS DEPOSITED ONTO THE SUPPORT MEDIA USING PHYSICAL VAPOR DEPOSITION in the names of Larry Brey et al., and filed Sep. 23, 2004; and in U.S. Ser. No. 11/275,416, filed Dec. 30, 2005, in the names of John T. Brady et al., titled HETEROGENEOUS, COMPOSITE, CARBONACEOUS CATALYST SYSTEM AND METHODS THAT USE CATALYTICALLY ACTIVE GOLD. The respective entireties of these two co-pending patent applications are incorporated herein by reference. The catalytic systems described in these patent applications provide excellent catalytic performance with respect to CO oxidation.
Titania that is nanoporous and/or nanosized is highly desirable as a support for a number of catalytic processes, including those incorporating catalytically active gold. Nanosized titania can be easily prepared by hydrolysis of titanium alkoxides, hydrolysis of titanium salts, and by gas phase oxidation of volatile titanium compounds. Thus, nanosized titania is readily available commercially at reasonable cost. In addition, titania in nanosized form can be readily dispersed in water or other solvents for application on other substrates and carrier particles and can be provided as a coating on a variety of substrates in nanoporous form.
Besides its availability in nanoporous and nanosized form, titania has surface properties that are amenable to strong catalytic effects. Titania is well known for its ability to form partially reduced surface structures comprising defect sites such as oxygen anion vacancies. The high density of oxygen anion vacancies provides sites for oxygen adsorption and the adsorbed oxygen has been shown to be mobile on titania, allowing the oxygen to be transported to active oxidation sites on catalysts comprising metal particles supported on titania (Xueyuan Wu, Annabella Selloni, Michele Lazzeri, and Saroj K. Nayak, Phys. Rev. B 68, 241402(R), 2003). Besides assisting in oxygen transport, the surface vacancies are known to help stabilize nanogold particles against deactivation through sintering and thus assist in enabling the generation of highly dispersed, catalytically active gold on titania catalysts. Titania has been found to be an excellent support for nanogold in highly active CO oxidation catalysts and for catalysts used for the direct epoxidation of propene (T. Alexander Nijhuis, Tom Visser, and Bert M. Weckhuysen, J. Phys. Chem. B 2005, 109, 19309-19319).
Nanogold on various substrates, including titania, has been proposed for use as a PROX catalyst. Although a number of methods have been examined, successful commercialization of a PROX catalyst using this approach has not occurred. An analysis of the situation is provided by Yu et al. (Wen-Yueh Yu, Chien-Pang Yang, Jiunn-Nan Lin, Chien-Nan Kuo and Ben-Zu Wan, Chem. Commun., 2005, 354-356):                Several reports in the literature have described the preferential oxidation of CO in a H2 rich stream over gold supported on TiO2. Among them, Haruta et al used a deposition-precipitation (DP) method, Choudhary et al used a grafting method, Schubert et at and Schumacher et al used impregnation and DP methods for the preparation of gold on the support. It was shown from their data that only a portion of CO in the feed stream was selectively oxidized to CO2 and none of the catalyst systems can achieve close to the expected 100% conversion.        
PROX catalysts comprising gold on nanoparticulate titania are described by Yu et al in the above referenced paper. But this work did not reveal a method by which the titania can be modified to show excellent PROX activity. As a result, the materials of Yu et al showed a strong sensitivity to carbon dioxide and moisture. The selectivity was very sensitive to changes in temperature and oxygen content and the challenge velocity had to be lowered in order to achieve modest PROX characteristics.
Mallick and Scurrell (Kaushik Mallick and Mike S. Scurrell, Applied Catalysis A, General 253 (2003) 527-536) reported that modifying titania nanoparticle substrates used for nanogold supports by hydrolyzing zinc onto the titania nanoparticles to form zinc oxide-coated titania nanoparticles caused a reduced catalytic activity for CO oxidation. The amount of zinc oxide introduced in this work, however, was excessive as compared to the required levels as shown herein. The work also did not reveal the improved PROX materials that could be prepared as shown herein.
Nanogold on nanoporous titania particles, however, has been found to be a potent catalyst for the reaction of hydrogen with oxygen. For example, Landon et al (Philip Landon, Paul J. Collier, Adam J. Papworth, Christopher J. Kiely, and Graham J. Hutchings, Chem. Commun. 2002, 2058-2059) have shown that catalytically active gold on titania could be used for the direct synthesis of hydrogen peroxide from H2 and O2. This high activity for hydrogen oxidation seemingly would make systems incorporating catalytically active gold deposited on nanoporous titania supports unsuitable for PROX applications. In the PROX applications, the catalyst system desirably oxidizes CO while avoiding hydrogen oxidation. Thus, while gold on titania has been examined as a PROX catalyst, commercial success for this application has been elusive.
Consequently, improvements are still desired for PROX catalysis. Notably, it is desirable to provide catalyst systems that show improved activity and selectivity for CO oxidation in the presence of hydrogen. It would also be desirable to provide catalyst systems that are relatively insensitive to the presence of carbon dioxide and water. Such catalyst systems would be very useful for removing CO from reformed hydrogen.